Many industrial processes may only be performed at low pressures. Such processes include, but are not limited to, semiconductor manufacturing, micro machining, vacuum deposition, and specialized coatings. In many such processes, pressure regulation is critical in order to maintain environmental conditions (e.g., temperature and pressure), a manometer, which is a type of pressure sensor, may be utilized. The manometer is configured to sense pressure changes in a process chamber that has reduced pressure.
One type of manometer that is sensitive enough to measure pressures as such low pressures is a diaphragm-based manometer. Diaphragm-based manometers are widely used in the semiconductor industry. In part, this is because diaphragm-based manometers are typically well suited to the corrosive services of this industry due to having high accuracy and being resistant to contamination. In particular, diaphragm-based manometers exhibit enhanced resistance to contamination and operate longer without maintenance.
A manometer serves as the vacuum/pressure sensing element and may be used to measure and/or control the pressure within a process chamber. A diaphragm-based manometer typically has a housing containing two chambers separated by a circular tensioned diaphragm. The first chamber is in fluid communication with the process chamber or other assembly in which the pressure is to be measured. The second chamber of the diaphragm-based manometer is commonly referred to as the reference chamber and is typically (although not necessarily) evacuated and sealed at a pressure that is substantially less than the minimum pressure the sensor senses.
A diaphragm, which is generally circular, is tensioned and separates the two chambers within a housing of the diaphragm-based manometer. The diaphragm is essentially formed of a thin metal that is mechanically constrained about its periphery. The diaphragm reacts to differential pressures by deforming into a bowed shape with the periphery remaining stationary. The diaphragm, thereby, serves as a flexing, grounded electrode. The diaphragm deforms as a reaction to the pressure difference across it and also interacts with electrostatic fields such that the deformation of the diaphragm may be resolved through these electrostatic interactions.
In close proximity to the diaphragm lies an electrode assembly. This assembly generally includes a stiff platform with a polished, electrically insulating surface that bears two conductive electrodes. The conductive electrodes are typically silk screen painted onto the surface, which is often a ceramic base. The configuration of the conductive electrodes of conventional diaphragm-based manometers generally includes an inner, solid circle and a ring that encircles the inner circle. The electrode assembly is mechanically constrained a fixed distance from the plane containing the periphery of the diaphragm so that the electrodes are very close to the diaphragm (<0.005 in) and run parallel to the surface of the diaphragm. Flexure of the diaphragm, due to applied pressure, can easily be computed by measuring the capacitance to ground at each electrode and subtracting one measurement from another.
Modern diaphragm-based manometers utilize two electrodes to monitor the flexure of the diaphragm. The capacitance to ground of the two electrodes (“common-mode capacitance”) varies with flexure of the diaphragm, but also changes with movement of the electrode assembly. Such movement occurs with temperature changes, temperature transients, and mechanical loading. Measurements using the difference in capacitance of the two plates (“difference capacitance”) are more stable since they reject motions between the diaphragm and electrodes and instead reflect the deflection of the diaphragm.
As the diaphragm is displaced, capacitance between the diaphragm and the conductive elements changes. The changing capacitance causes a change in charge being sensed from the two conductive elements, thereby providing a measurement to determine a change in pressure in the process chamber. The measured pressure change may be used for altering the environmental conditions by a controller of the vacuum chamber.
Diaphragm-based manometers are very precise. However, as a result, the manometers have components and tolerances that, too, are very precise. Generally speaking, there are a fair number of components in conventional diaphragm-based manometers that are used to (i) reduce temperature variation effects (e.g., thermal expansion and contraction) that impact repeatability of the manometer, (ii) reduce stray capacitance effects on measurement, (iii) reduce alignment variations of the components in the manometer, and so forth. As a result, the cost and production of the manometers are challenges that have plagued manufacturers for years. As an example, as thermal effects cause expansion and contraction of materials of the manometer, such as a metal housing and support hardware for the ceramic base, thermal cycles cause different readings at the same temperature on different sides of the thermal cycles as the materials themselves are slightly repositioned with respect to one another, even if formed of the same material, due to thermal conductivity rates not being identical. As understood, as a result of the production and repeatability issues, manufacturing diaphragm-based manometers is labor intensive and costly.